1. Field of the Invention
The present invention relates to a toner and a method for producing the same for use in an electrophotographic image forming apparatus such as a copier and a printer.
2. Description of the Related Art
Recently, an image forming apparatus for performing large quantity printing at a high speed has been developed. The image forming apparatus is required to continuously give a high-quality print image in an energy saving manner.
In order to continuously achieve a high-quality print image in an energy saving manner by a high-speed image forming apparatus, a toner having a lower Tg and a finer particle size is used. If the Tg of a toner is lower, the surface of a toner particle is softer, and the performing of large quantity printing (printing many sheets) causes an external additive attached to the surface of a toner particle to be embedded in the toner particle, thereby not enabling maintaining a stable charge amount. Therefore, it is difficult to continuously obtain a high-quality print.
In general, as additives for controlling a charge amount, silica and a charge controlling agent are attached to the surface of a toner. Collision between a toner and a toner or collision between a toner and a carrier is repeated during stirring of a developer to allow such additives attached to the surface to be separated from the surface of a toner and to be embedded in a toner mother material, and thus such external additives cause decrease in charge giving function to result in a toner having a low charge amount. Deteriorations in chargeability and transferability cause image defects such as variation in image density.
Currently, full coloration of an image proceeds, and thus there is a demand for dealing with various printing patterns. For example, there is a higher need for such an image that a color part is incorporated in a black letter image in a small amount, rather than a photographic image using all colors in an office. In such a case, while a toner of a certain color having a high print percentage is consumed in a large amount, with respect to another color having a low print percentage, a fresh toner thereof is slowly consumed and thus the stirring time of a developer in a developing unit is significantly increased, thereby remarkably causing an additive on the surface of a toner to be separated and an external additive to be embedded to a toner mother material.
In particular, a toner having a lowered Tg easily causes an external additive to be embedded due to the softness of a resin, to result in toner aggregation and a weakly charged toner, thereby causing image defects such as variation in image density.
As a measure for preventing toner aggregation, a technique is disclosed in which a large size external additive is added and a toner having a core-shell structure whose shell has a high Tg is used (see, e.g., Japanese Patent Application Laid-Open No. 2001-175025).
As a measure for preventing an external additive from being embedded, a technique is disclosed in which a plurality of inorganic particles having mutually different particle sizes (for example, large size external additives and small size external additives) are used to give a spacer function to large size external additives, thereby attempting to stabilize developability and transferability (see, e.g., Japanese Patent Application Laid-Open No. 2006-39023).
As an additional measure for preventing an external additive from being separated, a technique is disclosed in which a plurality of external additives having mutually different shapes and work functions besides the different particle size are used to prevent an external additive from being separated from toner base particles, thereby stabilizing chargeability. The measure is specifically described with reference to FIGS. 4A to 4D. Toner base particles 8a is constituted by an external additive 12 externally added. For the external additive 12, two types of hydrophobic negatively chargeable silicas (SiO2) 13 and 14 having different average primary particle sizes, small particle size and large particle size, and hydrophobic rutile anatase type titanium dioxide (TiO2) 15 are used, respectively. In this case, the hydrophobic negatively chargeable silica 13 constitutes a first external additive, and the hydrophobic rutile anatase type titanium dioxide 15 constitutes a second external additive. Herein, the rutile anatase type titanium dioxide 15 has a spindle shape having a longer axis diameter and a shorter axis diameter, and in a negatively charged toner 8, the longer axis diameter of the rutile anatase type titanium dioxide 15 is set to be greater than the average primary particle size of the negatively chargeable silica 13 and 14.
Thus, the rutile anatase type titanium dioxide 15 is certainly attached to the toner base particles 8a via the negatively chargeable silica 13 and thus is hardly separated from the toner base particles 8a. Then, the toner base particles 8a is negatively charged by the negatively chargeable silica 13 and 14, and prevented from being excessively charged by the rutile anatase type titanium dioxide 15. Thus, charge characteristics are stabilized for a long period, and durability and transfer efficiency are enhanced (see, e.g., Japanese Patent No. 4141721).
However, in the above conventional methods for suppressing the separation of the additives from the surface of the toner and the embedding of the external additive in the toner mother material, an external pressure is applied to the toner particle by collision between toners or between a toner and a carrier or by parts in the developing unit, such as a blade, to thereby cause the external additive to be embedded in the toner mother material or to be separated from the surface of the toner mother material particle under the condition where a large number of manuscripts having various print percentages, in particular, manuscripts having a low print percentage (for example, print percentage: 1%) are printed in a high-speed image forming apparatus or under the printing condition where a toner remains in the developing unit for a long time because of having a low print percentage. Accordingly, the charging function of the external additive to the toner is made constant and also the spacer effect by the external additive is difficult to be stably maintained. Therefore, both of charge characteristics and transfer characteristics are difficult to be maintained over a long period, thereby causing deteriorations in chargeability, developability, transferability and the like. As a result, there is such a problem that deterioration in image quality, such as reduction in image density, is caused.